This invention relates to a speaker system provided with a passive radiator to be mounted in a vehicle. Such a speaker system is typically mounted on the rear window tray or shelf of an automobile. Modern automobiles are frequently equipped with stereo FM cassette players or other hi-fi sound systems. However, in view of space constraints large-full range speakers cannot be used. Hence, small speakers are employed with attendant deficiencies in frequency response.
In the case of a speaker cabinet in which a compact speaker unit is mounted, the reproduction of the lower sound spectrum tends generally to be deficient over the acoustic reproduction frequency range due to various restrictions of the cabinet design. Such speakers tend to sound "thin" because of a bias to reproduce high frequencies of the audio spectrum. In order to improve the deficiency of the lower sound spectrum, a passive radiator (or drone cone) type speaker system has been proposed. As shown in FIG. 1, a speaker unit B and a passive radiator C are provided in the same housing A. The passive radiator C does not have a magnetic circuit. The passive radiator C is adapted to resonate at frequencies lower than those reproduced in the speaker unit A, so that the reproduction range of the lower frequency spectrum can be broadened. The resonance of the passive radiator C takes place via air in the same phase as the speaker unit B.
Referring again to the speaker system of the type mounted on the rear window tray or shelf of the automobile, the size of the speaker cabinet and the speaker unit are restricted due to the area of the mounting place of the automobile. Modern automobiles generally have sloping rear windows and narrow rear shelves or trays. For most cases, the available speaker system is such that the volume of the speaker cabinet is at most about 3 liters. In such a size, the passive radiator type speaker system is preferable in terms of cost for reproducing the lower sound spectrum.
In FIG. 1, if a horn (not shown) is attached to the opening portion of the passive radiator C, the radiating sound energy can be effectively diffused in a certain direction to establish directionality of sound emission. As a result, the lower sound spectrum can be further strengthened. In the rear window tray or shelf of the automobile, however, it is extremely difficult to attach the horn to establish this effect because of the above noted positional or shelf restriction.